Understanding the behavior of ferroelectric materials at the nanoscale dimension is of importance to the development of molecular electronics, in particular for random access memory (RAM) and logic circuitry. Indeed, transition metal oxides with a perovskite structure are noteworthy for their advantageous dielectric, piezoelectric, electrostrictive, pyroelectric and electro-optic properties with the corresponding applications in the electronics industry for pyroelectronic detectors, imaging devices, optical memories, modulators, deflectors, transducers, actuators, and high-k dielectric constant materials. Such properties and applications for perovskite oxides are described, for example, in N. A. Hill, J. Phys. Chem. B, 2000, 104, 6694, J. F. Scott, Ferroelectr. Rev., 1998, 1, 1, and A. J. Millis, Nature, 1998, 392, 147.
As reported by T. K. Song, et al., Solid State Commun., 1996, 97, 143, the perovskite oxides including, for example, BaTiO3 and SrTiO3, typically exhibit nonlinear optical coefficients and large dielectric constants. Because these effects are dependent on the metallic elemental ratios, impurities, microstructure and finite size, considerable effort has been expended in the controllable synthesis of crystalline materials and thin films of these ferroelectric oxides. See, in this regard, L. A. Willis, et al., AppL. Phys. Lett., 1992, 60, 41, J. Zhang, et al., AppL. Phys. Lett., 1994, 64, 2410, J. Zhao, et al., J. Mater. Chem., 1997, 7, 933, and X. W. Wang, et al., Mater. Sci. Eng. B, 2001, 86, 29.
One-dimensional nanotube/nanowire systems offer fundamental scientific opportunities for investigating the influence of size and dimensionality of materials with respect to their collective optical, magnetic and electrochemical properties. S. O'Brien, et al., J. Am. Chem. Soc., 2001, 123, 12085 have reported the fabrication of monodispersed nanoparticles of barium titanate with diameters ranging from 6 to 12 nm. The syntheses of barium titanate nanoparticles have also been described, for example, in Niederberger et al., Angew. Chem. Intl. Ed., v. 43, 2270 (2004).
Additionally, BaTiO3 and SrTiO3 nanorods, which are not hollow, have been fabricated by solution-phase decomposition of bimetallic alkoxide precursors in the presence of coordinating ligands, yielding well-isolated nanorods with diameters ranging from 5 to 60 nm and lengths up to >10 microns. The fabrication of nanorods using the solution-phase decomposition method has been described, for example, in J. J. Urban, et al., J. Am. Chem. Soc., 2002, 124, 1186 and W. S. Yun, et al., Nano Lett., 2002, 2, 447. It is evident, from these studies, that the structures of the barium, strontium, and titanium precursors used play an important role in determining the composition, particle size and monodispersity, morphology, and properties of the final product.
In addition to fabricating nanorods, the prior art also includes various methods of fabricating nanotubes. Nanotubes differ from nanorods because nanotubes typically have a hollow cavity, whereas nanorods are completely filled nanomaterials. BaTiO3 and PbTiO3 nanotubes have been developed using a sol-gel template synthesis process. Such a process is described, for example, by B. A. Hernandez, et al., Chem. Mater., 2002, 14, 480, and B. A. Hernandez, et al., J Korean Chem. Soc., 2002, 46, No. 3, 242. A sol-gel electrophoresis synthesis method for fabricating barium titanate nanorods is described, for example, in Adv. Funct. Mater., v. 12(1), 59. The prior art sol-gel template process produces hollow nanotube bundles that have an outer diameter of 200 nm and a length of about 50 μm. The nanotube bundles produced using the sol-gel template process disclosed by B. A. Hernandez, et al. are neither isolated nanotubes nor ordered arrays, but instead are ‘spaghetti-like’ tangles that cannot be used for molecular electronic applications.
Other techniques besides the sol-gel template synthesis process disclosed in the Hernandez, et al. references have also been employed in fabricating ferroelectric nanotubes. F. D. Morrison, et al., Los Alamos National Laboratory, Preprint Archive, Condensed Matter (2003), pp. 1–19 describe the fabrication of ferroelectric nanotubes using misted chemical solution deposition (mCVD) and pore wetting.
U.S. Patent Publication 2003/0026985 A1 to Griener, et al. describes the formation of hollow fiber nanotubes having an internal diameter from 1 nm to 100 nm. The hollow nanotubes of this reference are produced by coating a degradable material with a non-degradable material and then degrading the degrading material. The prior art nanotubes disclosed in the Griener, et al. publication are biphasic and have significant amounts of amorphous impurities due to the nature in which the nanotubes are produced. By “biphasic”, it is meant that the prior art nanotubes contain two very different materials with distinctive properties having a defined interface separating the two distinctive substances.
In many of the prior art methods described above, organometallic precursors, which are extremely toxic, expensive, unstable, explosive and/or pyrophoric, are employed. In addition, many of the prior art methods require that a high-temperature annealing process be used. As such, the prior art methods of fabricating perovskite nanotubes include harsh reaction conditions that may have an adverse effect on the resultant nanotubes.
In view of the drawbacks mentioned with the prior art methods of fabricating perovskite nanotubes, there is a continued need for providing a relatively simple and cost effective means for fabricating monophasic perovskite nanotubes that include mild, low-temperature solution conditions.